Take telephone scene as an example, 

1. You can show both side of the characters talking and cut between them when they are changing lines. In this way audience can see and hear at the same time, it’s a easy way to understand what is going on

2. You can just show one person’s face for the entire time, and keep the other one off-screen. But we can hear the voice of the other one. In this way, audience will be interested in what the other character’s interests, motives, is he/she alone, what is she doing?…

3. You also just show one person’s face and voice broken by pauses. In this way, audience will keep lots of attention on the character’s expression and behaviors, and wondering what the other character said according to the character’s behavior.

sound shapes our understanding of images

Sound is the background of our visual world. ‘Chris Marker demonstrates the power of sound to alter our understanding what’s onscreen in his film, Letter from Siberia’. He shows the same image with different soundtrack, it turns out there is a big difference of what it brings to audience. ‘The audience will construct the same images differently, depending on the voice-over commentary.

sound direct our attention

When we hear something, we will think immediately about what happened there. In filmmaking, if the sound appears, then it cuts to next shot, we can see the answer, sometimes it is not what exactly we thought it is. Then we will think again. ‘Horror and mystery scenes often use a sound of an unseen source to engage audience’s interest, but all types of scenes can take advantage of that possibility. In addition, sound gives a new value to silence. A quiet passage in a film can create almost unbearable tension, forcing the viewer to concentrate on the screen.

reference: Bordwell,D & Thompson,K, FILM ART AN INTRODUCTION, Sound in cinema, pp.266-270.